Chair base



FIG. 1 is a top plan view showing a chair base which forms my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a front elevational view thereof; and,

FIG. 4 is a left side elevational view thereof, this side and the opposite right side elevational view being mirror images. 

The ornamental design for a chair base, as shown and described. 